std::ranges::view, std::ranges::enable_view, std::ranges::view_base

 

C++

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Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_trange_value_trange_reference_t	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)	elements_of (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Range factories
empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (C++23)(C++23)	basic_istream_viewviews::istream
Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23) filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)
Range generators
std::generator (C++23)
Range adaptor objects
Range adaptor closure objects
range_adaptor_closure (C++23)
Helper items
copyable-box movable-box (until C++23)(C++23)	simple-view non-propagating-cache

 

Defined in header <ranges>
template<class T> concept view = ranges::range<T> && std::movable<T> && ranges::enable_view<T>;	(1)	(since C++20)
template<class T> inline constexpr bool enable_view =     std::derived_from<T, view_base> || /*is-derived-from-view-interface*/<T>;	(2)	(since C++20)
struct view_base { };	(3)	(since C++20)

1) The view concept specifies the requirements of a range type that has suitable semantic properties for use in constructing range adaptor pipelines.

2) The enable_view variable template is used to indicate whether a range is a view. /*is-derived-from-view-interface*/<T> is true if and only if T has exactly one public base class ranges::view_interface<U> for some type U, and T has no base classes of type ranges::view_interface<V> for any other type V.
Users may specialize enable_view to true for cv-unqualified program-defined types which model view, and false for types which do not. Such specializations must be usable in constant expressions and have type const bool.

3) Deriving from view_base enables range types to model view.

Semantic requirements

1) T models view only if:

• move construction of T has constant time complexity, and
• if \(\scriptsize N\)N copies and/or moves are made from a T object holding \(\scriptsize M\)M elements, then these \(\scriptsize N\)N objects have \(\scriptsize \mathcal{O}{(N+M)}\)𝓞(N+M) destruction (which implies that a moved-from view object has \(\scriptsize \mathcal{O}{(1)}\)𝓞(1) destruction), and
• either std::copy_constructible<T> is false, or copy construction of T has constant time complexity, and
• either std::copyable<T> is false, or copy assignment of T has no more time complexity than destruction followed by copy construction.

Notes

Examples of view types are:

• A range type that wraps a pair of iterators, e.g., std::ranges::subrange<I>.
• A range type that holds its elements by std::shared_ptr and shares ownership with all its copies.
• A range type that generates its elements on demand, e.g., std::ranges::iota_view.

A copyable container such as std::vector<std::string> generally does not meet the semantic requirements of view since copying the container copies all of the elements, which cannot be done in constant time.

While views were originally described as cheaply copyable and non-owning ranges, a type is not required to be copyable or non-owning for it to model view. However, it must still be cheap to copy (if it is copyable), move, assign, and destroy, so that range adaptors will not have unexpected complexity.

By default, a type modeling movable and range is considered a view if it is publicly and unambiguously derived from view_base, or exactly one specialization of std::ranges::view_interface.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P2325R3	C++20	view required default_initializable	does not require
LWG 3549	C++20	enable_view did not detect inheritance from view_interface	detects
P2415R2	C++20	the restriction on the time complexity of destruction was too strict	relaxed